1. Field of the Invention
The present invention relates to a high-performance tire, which mitigates the aqua planing effect and increases wear-resistance, as well as to the manufacturing method and mold thereof.
More particularly, the tire of the invention is of the type comprising a tread band provided with at least one large-sized, extended circumferential cavity, preferably but not exclusively disposed at a central position, for quickly ejecting water collected on the ground so as to counteract the effect currently referred to as "aquaplaning". Aquaplaning occurs when a vehicle loses ground contact, particularly at high speed, due to the continuous water film interposed between the tire and the ground.
Tires of the above type, in which the tread substantially comprises two distinct axial portions separated from each other by a wide circumferentially-extended cavity, are well-known and have been described in several patents briefly mentioned hereinafter.
2. Description of the Related Art
For instance, patent EP 0 503 404 discloses a tread comprising circumferential and transverse S-shaped grooves, crossed with each other so as to form a plurality of blocks, and provided with a central cavity of a width corresponding to 10%-20% of the tread width and a depth corresponding to 78%-80% of the overall tread thickness.
Patent EP 0 503 405, in addition to the above, gives particular values for the angle of inclination of the entrance and exit walls of the blocks relative to the ground-contacting area. Patent EP 0 503 406 also illustrates particular connecting arcs of the side walls of the different tread band blocks.
Other details concerning the geometrical and operating features of the grooves and blocks in the tread pattern are described in U.S. Pat. Nos. 5,176,766; 4,700,762; and 4,785,863.
U.S. Pat. No. 4,796,683 refers to a tread pattern comprising a circumferentially-directed cavity and transverse grooves of great inclination to the cavity so as to promote water draining. In this tread pattern blocks are formed that have acute and obtuse angles. Close to the acute angles where stiffness is lower, the greatest abrasion can be found. Elimination of this drawback is suggested by forming particular chamfers in the blocks so as to increase the block stiffness at aimed areas. U.S. Pat. No. 4,289,182 refers to a motorcycle tire adapted for high speed and provided with a tread pattern having a circumferentially-directed cavity and two sets of rectilinear transverse grooves at each side of the cavity. Between the grooves of the two sets, a zigzag rib is formed.
The Applicant has noticed that the known tires of the above type generally exhibit a marked wear of the tread band at the central area thereof, close to the cavity along the equatorial plane, which takes place early and continues over time. The remaining tread portions outside of the central area exhibit slower wear, and therefore, the overall lifetime of the tire is reduced as compared to possible lifetime of an evenly wearing tire.
It is a current hypothesis that this type of wear depends on a greater flexibility and deformability of the carcass by effect of the wide central cavity in the tread pattern.
U.S. Pat. No. 4,687,037, directed to a tire comprising a carcass, a belt and a tread band having a circumferential cavity in the middle, the width of which corresponds to 20% of the tread width, attempts to find a solution to this problem by imparting a particular rigidity to the belt. Actually the belt comprises a plurality of reinforcing layers having cords crossed with each other following known techniques, partly substantially extended over the whole width of the tread band, and partly axially concentrated on given preferential areas.
In particular, the belt structure has four layers of mutually crossed cords of a width corresponding to the tread band width and two further reinforcing layers of a width as large as or slightly larger than the width of the central cavity. These further layers of reduced width are disposed below the cavity, the first layer between the two pairs of layers of a width as large as that of the tread band and the second layer immediately under the cavity, respectively.
Radially projecting outwardly from the cavity bottom, in such a manner as to nearly constitute a link between the two distinct tread band portions, there is a rib of elastomeric material extending over the whole cavity length.